Pressure Support Ventilation in Neurosurgical Patients: Can We Safely Reduce Assistance? Evaluation of Neurosurgical Patients' Ventilation Distribution - The ENVISION Study.

OBJECTIVES: To identify the prevalence of over-assistance from mechanical ventilation (MV) and to assess whether reducing MV support could be done safely in neurosurgical ICU patients in terms of risk of under-assistance and brain's oxygenation. DESIGN: Prospective observation study. SETTING: Neurosurgical trauma ICU, Toronto, ON, Canada. PATIENTS: Twenty-seven brain-injured patients on MV having indication of a spontaneous breathing trial (SBT). INTERVENTIONS: Level of pressure support ventilation (PSV). MEASUREMENTS AND MAIN RESULTS: In neurosurgical patients, regional ventilation distribution using electrical impedance tomography, patient's respiratory drive (airway occlusion at 100 ms [P0.1]), respiratory muscle pressure (Pmus), diaphragm and parasternal intercostal (PI) thickening fraction, brain oximetry, and electroencephalogram were assessed at clinical PSV (ClinPS), low PSV (LowPS, pressure support [PS] 5 cm H2O, positive end-expiratory pressure [PEEP] 5 cm H2O), SBT, PS 0 cm H2O, and PEEP 0 cm H2O. Over-assistance was defined by pressure muscle index less than 0 cm H2O; under-assistance was defined as Pmus greater than or equal to 15 cm H2O. Mixed effects models were used for analysis. Imbalanced dorsal/ventral distribution of ventilation improved by reducing assistance while respiratory effort increased. Over-assistance was present in ten cases (37%) during ClinPS and in none at LowPS and SBT; under-assistance was present in two, four, and seven cases at ClinPS, LowPS, and SBT. During SBT, compliance and end-expiratory lung volume decreased (p < 0.0001). Brain activity did not vary. P0.1 greater than or equal to 4 cm H2O was associated with Pmus greater than or equal to 15 cm H2O with 80% sensitivity and 91% specificity during SBT. CONCLUSIONS: Neurosurgical patients seem to frequently be overassisted under PSV. Reducing the ventilatory support is often feasible and Pmus and P0.1 can help with detecting under-assistance.